1. Field of the Invention
The present invention relates to a photo receiver and a photo receiving method, and more particularly, to a signal cutoff detector and a signal cutoff detecting method.
2. Description of the Related Art
For enhancing reliability of an optical communication system to quickly take countermeasure in case a failure occurs, demanded is improving a failure detection function of each part of the system.
For such a purpose, a photo receiver is conventionally provided with a function of detecting a signal being cut off. Structure of a conventional signal cutoff detection circuit is shown in FIG. 12. The signal cutoff detection circuit of FIG. 12 includes a photo detector 10, a preamplifier 11, an amplifier or limiter amplifier 12 with an automatic gain control (AGC) function, a band-pass filter 13, an amplifier 14, a limiter amplifier 15, a peak value detection circuit 16, a delay element 17, a discrimination circuit 18 and a comparator 19. A light signal applied to the photo receiver is converted into an electric signal by the photo detector 10 and then amplified into a signal of a predetermined level by the preamplifier 11 and the AGC amplifier or limiter amplifier 12. An output of the AGC amplifier or limiter amplifier 12 is branched into two, one of which is applied to a data input terminal of the discrimination circuit 18. The other signal is input to the band-pass filter 13, in which a clock component contained in the data signal is extracted. After being amplified to a signal of a predetermined level by the amplifier 14 and the limiter amplifier 15, the extracted clock signal is input to the discrimination circuit 18, in which it is used as an identification clock of a data signal. A delay difference between a clock signal and a data signal is compensated for by the delay element 17.
Here, a part of the output of the amplifier 14 is input to the peak value detection circuit 16. At the peak value detection circuit 16, a peak level of the input clock signal is detected and the comparator 19 determines whether a clock signal exists or not based on whether the detected level reaches a predetermined level. When the determination is made that no clock signal exists, it can be known that no appropriate light signal is input to the photo receiver or that some abnormality occurs in the photo receiver.
In the field of optical communication today, there are many cases where an optical relay amplifier is used as a relay and where wave-length division multiplexing (WDM) transmission is conducted for increasing a transmission capacity per optical fiber.
In such a transmission system, in a case where a failure on a transmission path, a photo transmitter and receiver or a relay eliminates a light data signal, an amplified spontaneous emission (ASE) generated at each optical relay amplifier is accumulated to reach a photo receiver. On the other hand, in wave-length division multiplexing (WDM) transmission, when a light data signal of a part of wave-length channels is lost due to some failure or other, processing of superposing a dummy signal (continuous wave (CW) light) on a wave-length whose signal is lost called xe2x80x9ckeep alivexe2x80x9d is conducted in some cases in order to balance power between the respective wave-length channels. In such cases, while wave-length light itself exists, no signal exists, so that such cases should be detected being abnormal. At the time of a noise caused by ASE or the time of keep alive, a photo receiver might be subjected to a high-level noise, which makes it difficult for conventional methods to detect signal cutoff.
An object of the present invention is to provide a signal input cutoff detector having a function of reliably detecting signal cutoff in a wave-length division multiplexing (WDM) transmission system and a transmission system including an optical relay amplifier.
According to the first aspect of the invention, a signal input cutoff detector comprises
a signal cutoff detector which detects a level of a data signal converted from a light input signal into an electric signal and when the level is below a predetermined value, generates a first alarm signal; and
an out-of-synchronization detector which monitors a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and when a level of the control signal exceeds a predetermined value, generates a second alarm signal.
In the preferred construction, the signal input cutoff detector further comprises an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal.
In another preferred construction, the signal cutoff detector includes
an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and
a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal.
In another preferred construction, the signal input cutoff detector further comprises an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal.
In another preferred construction, the signal input cutoff detector further comprises an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal,
the delay time being half a time length of one bit of the data signal.
In another preferred construction, the signal input cutoff detector further comprises an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal,
the delay time being half a time length of one bit of the data signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal,
the at least one predetermined reference voltage including a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the signal input cutoff detector further comprises an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal,
the at least one predetermined reference voltage including a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal,
the at least one predetermined reference voltage including a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and
a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal,
the delay time being half a time length of one bit of the data signal, and
the at least one predetermined reference voltage including a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the signal cutoff detector further including a hysteresis amplifier provided at an input part of the signal cutoff detector.
In another preferred construction, the signal input cutoff detector further comprising an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal cutoff detector further including a hysteresis amplifier provided at an input part of the signal cutoff detector.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and which further comprises
a hysteresis amplifier provided at an input part of the signal cutoff detector.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and a hysteresis amplifier provided at an input part of the signal cutoff detector,
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and a hysteresis amplifier provided at an input part of the signal cutoff detector,
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal,
the delay time being half a time length of one bit of the data signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and a hysteresis amplifier provided at an input part of the signal cutoff detector,
the at least one predetermined reference voltage including a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the out-of-synchronization detector including a second comparator which compares a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputs the second alarm signal.
In another preferred construction, the signal input cutoff detector further comprising an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal,
wherein the out-of-synchronization detector including a second comparator which compares a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputs the second alarm signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal, and
the out-of-synchronization detector including a second comparator which compares a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputs the second alarm signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the out-of-synchronization detector including a second comparator which compares a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputs the second alarm signal,
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the out-of-synchronization detector including a second comparator which compares a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputs the second alarm signal,
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal, and
the delay time being half a time length of one bit of the data signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculats an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the out-of-synchronization detector including a second comparator which compares a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputs the second alarm signal,
the at least one predetermined reference voltage including a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the signal cutoff detector further including a hysteresis amplifier provided at an input part of the signal cutoff detector, and
the out-of-synchronization detector including a second comparator which compares a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputs the second alarm signal.
According to the second aspect of the invention, a photo receiver comprises
a photo detector which converts a light input signal into an electric signal,
an amplifier which amplifies the electric signal to have a predetermined amplitude,
a frequency phase-locked loop which contains a VCO and generates a clock synchronized with an output of the amplifier and a discrimination circuit for discriminating an output of the amplifier by the clock, and
a signal input cutoff detector, wherein
the signal input cutoff detector including
a signal cutoff detector which detects a level of a data signal converted from a light input signal into an electric signal and when the level is below a predetermined value, generates a first alarm signal, and
an out-of-synchronization detector which monitors a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and when a level of the control signal exceeds a predetermined value, generates a second alarm signal.
In the preferred construction, the signal input cutoff detector further including an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal.
In another preferred construction, the signal input cutoff detector further including an alarm processor which generates a third alarm signal according to generation conditions of the first alarm signal and second alarm signal,
the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal, and
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal, and
the autocorrelation detector including a delay element which delays the data signal by a predetermined delay time to output a delayed data signal, an exclusive OR circuit which calculates an exclusive OR of the data signal and the delayed data signal to output an exclusive OR signal, and a first integrator which calculates a mean value of the exclusive OR signal,
the delay time being half a time length of one bit of the data signal.
In another preferred construction, the signal cutoff detector including an autocorrelation detector which calculates an autocorrelation of the data signal to output an autocorrelation signal, and a first comparator which compares the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputs the first alarm signal,
the at least one predetermined reference voltage including a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the signal cutoff detector further including a hysteresis amplifier provided at an input part of the signal cutoff detector.
In another preferred construction, the out-of-synchronization detector including a second comparator which compares a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputs the second alarm signal.
According to the third aspect of the invention, a signal input cutoff detecting method comprising the steps of:
detecting a level of a data signal converted from a light input signal into an electric signal and when the level is below a predetermined value, generating a first alarm signal, and
monitoring a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and when a level of the control signal exceeds a predetermined value, generating a second alarm signal.
In the preferred construction, the signal input cutoff detecting method further comprising
alarm processing step of generating a third alarm signal according to generation conditions of the first alarm signal and second alarm signal.
In another preferred construction, the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal.
In another preferred construction, the signal input cutoff detecting method further comprising
alarm processing step of generating a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal.
In another preferred construction, the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal,
the at least one predetermined reference voltage including
a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and
a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the signal input cutoff detecting method further comprising
alarm processing step of generating a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal,
the at least one predetermined reference voltage including
a first reference voltage having a value between the mean value when the data signal is large enough and zero, and
a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the out-of-synchronization detection step includes
second comparison step of comparing a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputting the second alarm signal.
In another preferred construction, the signal input cutoff detecting method further comprising
alarm processing step of generating a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the out-of-synchronization detection step includes
second comparison step of comparing a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputting the second alarm signal.
In another preferred construction, the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal, and
the out-of-synchronization detection step includes
second comparison step of comparing a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputting the second alarm signal.
In another preferred construction, the signal input cutoff detecting method further comprising
alarm processing step of generating a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal, and
the out-of-synchronization detection step includes
second comparison step of comparing a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputting the second alarm signal.
In another preferred construction, the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal,
the at least one predetermined reference voltage including
a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and
a second reference voltage higher than the mean value obtained when the data signal is large enough, and
the out-of-synchronization detection step includes
second comparison step of comparing a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputting the second alarm signal.
In another preferred construction, the signal input cutoff detecting method further comprising
alarm processing step of generating a third alarm signal according to generation conditions of the first alarm signal and second alarm signal, wherein
the signal input cutoff detection step includes:
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal,
the at least one predetermined reference voltage including
a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and
a second reference voltage higher than the mean value obtained when the data signal is large enough, and
the out-of-synchronization detection step includes
second comparison step of comparing a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputting the second alarm signal.
According to another aspect of the invention, a photo receiving method comprising
step of converting a light input signal into an electric signal and amplifying the converted signal to have a predetermined amplitude,
step of, with a VCO contained, generating a clock synchronized with a data signal amplified to the predetermined amplitude,
step of discriminating a data signal amplified to the predetermined amplitude by the clock,
signal input cutoff detection step of detecting a level of a data signal converted from a light input signal into an electric signal and when the level is below a predetermined value, generating a first alarm signal, and
out-of-synchronization detection step of monitoring a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and when a level of the control signal exceeds a predetermined value, generating a second alarm signal.
In the preferred construction, the photo receiving method further comprising
alarm processing step of generating a third alarm signal according to generation conditions of the first alarm signal and second alarm signal.
In another preferred construction, the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal.
In another preferred construction, the signal input cutoff detection step includes
autocorrelation detection step of calculating an autocorrelation of the data signal to output an autocorrelation signal, and
first comparison step of comparing the autocorrelation signal with at least one predetermined reference voltage and when the autocorrelation signal is larger, outputting the first alarm signal,
the at least one predetermined reference voltage including
a first reference voltage having a value between the mean value obtained when the data signal is large enough and zero, and
a second reference voltage higher than the mean value obtained when the data signal is large enough.
In another preferred construction, the out-of-synchronization detection step includes
second comparison step of comparing a VCO control signal which controls a voltage controlled oscillator (VCO) to have a frequency and a phase synchronized with the data signal and a predetermined reference voltage and when the VCO control signal is larger, outputting the second alarm signal.
As described in the foregoing, the present invention is provided with a signal cutoff detector for detecting a level of an input data signal and when the level is lower than a predetermined value, generating a first alarm signal and an out-of-synchronization detector for generating a second alarm signal when a VCO control signal from a frequency phase-locked loop exceeds a predetermined value, that is, when the signal is out of synchronization. Using these two detectors to take detection results of each detector into consideration enables reliable generation of an alarm against a failure, as well as enabling a failing part to be cut out.
Other objects, features and advantages of the present invention will become clear from the detailed description given herebelow.